1. Field of the Invention
The present invention relates to a catalyst for production of a polyester, a process for producing a polyester using such a catalyst and a titanium-containing polyethylene terephthalate having excellent characteristics.
2. Description of the Background
Heretofore, a polyester represented by a polyethylene terephthalate is excellent in mechanical strength, chemical stability, gas barrier properties, hygienic properties, etc., is available at a relatively low cost and is light in weight, and thereby has been widely used as various packaging materials as e.g. bottles and films, or as fibers, etc.
Such a polyester has been produced mainly by employing an antimony compound as a polycondensation catalyst. However, because of problems of foreign matters/haze due to deposition of antimony metal and such a problem that the antimony compound is required as a catalyst in such a large amount of from 300 to 400 ppm, a highly active catalyst has been strongly desired.
As a countermeasure, as a process for producing a polyester resin containing no antimony compound, many processes employing a Group 4B compound, particularly a titanium compound as a polycondensation catalyst have been proposed.
As a titanium compound used as a polycondensation catalyst, the following titanium compounds have been proposed for example.
It has been known to use a coprecipitate alone or as a mixture, which is produced by simultaneous hydrolytic precipitation of a titanium compound and a metal compound of a metal selected from Groups IA, IIA, VIIIA, IB, IIB, IIIB and IVB of the Periodic Table. Specifically, it has been known to use as a catalyst a coprecipitate produced by simultaneous hydrolysis of titanium tetraisopropoxide and an alkoxy compound of e.g. magnesium for example with water in dehydrated ethanol (JP-A-2002-503274).
Preparation of the above catalyst requires a plurality of steps including hydrolysis of an alkoxy compound, centrifugal separation, washing and drying. The catalyst thus prepared is once dried and then subjected to a polycondensation reaction as a slurry of a solvent such as ethylene glycol, and accordingly, the catalyst comprises coarse particles, and the catalyst may remain as a foreign matter in a polyester to be obtained in some cases.
Further, it has been known to use as a catalyst a mixture obtained by preliminarily mixing a composite oxide comprising titanium and silicon as main metal elements with a diol or its ester forming derivative, followed by heating at from 160 to 220° C. Specifically, it has been disclosed to use as a catalyst a product obtained by hydrolyzing a mixture of titanium tetraisopropoxide and ethyl orthosilicate with water in ethanol to obtain a coprecipitate, and subjecting the coprecipitate to heat treatment at 198° C. in ethylene glycol (JP-A-2001-288262).
By use of the above catalyst, the amount of foreign matters in a polyester resin tends to be small as compared with a case of using antimony as a catalyst, but according to studies by the present inventors, the above catalysts has been found to be insufficient for practical use. Further, preparation of the above catalyst also requires a plurality of steps including mixing of catalyst components, heating for formation of a gel and vacuum drying.
Further, it has been known to use as a promoter a polysiloxane for polymerization for the purpose of improving the color tone. Specifically, it has been disclosed to use an organic polyborosiloxane (JP-A-59-142221). The polysiloxane disclosed in the publication is obtained by mixing diphenyl dichlorosilane with boric acid and then reacting them for 12 hours, and preparation of the catalyst takes long.
Further, it has been known to add at the time of polymerization a titanium compound and at least one compound selected from a magnesium compound, an aluminum compound, a barium compound and the like. Specifically, it has been disclosed to add titanium tetrabutoxide and magnesium acetate at the time of polymerization (JP-A-2000-143789).
The method disclosed in the above publication has such problems that a polyester to be obtained has poor color tone, and the acetaldehyde content after solid phase polymerization is high.
Further, there is some restriction for use in some cases, for example, when a plurality of metal compounds among the above catalysts are used in combination as a catalyst, it is required to specify the timing of addition of each metal compound.
Further, a polyester produced by using a titanium compound as a polycondensation catalyst is insufficient in thermal stability as represented by an acid value or the like, as compared with a polyester resin produced by using an antimony compound in some cases.
However, according to studies by the present inventors, it has been found that a method of employing such a catalyst comprising a titanium compound is not satisfactory as an industrial production method from the following reasons. That is, since the polycondensation reaction takes long, a decomposition reaction will proceed, and the color tone of a polyester to be obtained tends to deteriorate, thermal stability as represented by an acid value or the like may be insufficient, the volume resistivity may be insufficiently high, and the acetaldehyde content in the resin and the foreign matter content in the polymer are not satisfactory.
Accordingly, an alternative catalyst which has excellent activity, with which a high quality polyester can be produced and which can easily be handled, has been strongly desired.